Data storage systems, such as disc drives, typically store information on surfaces of storage media such as magnetic or optical discs. In a typical disc drive, one or more discs are mounted together on a spindle. The spindle causes the disc(s) to spin and the data surfaces of the disc(s) to pass under respective hydrodynamic and aerodynamic bearing sliders. Each slider is typically mounted on an actuator arm that moves over each disc surface.
When information is stored on a disc it is generally stored in a set of concentric data tracks. The tracks on the disc surface are typically divided into data sectors. Data sectors are the basic units of data storage on a disc surface. A sector is a “pie-shaped” angular section of a track that is bounded on two sides by radii of the disc and on the other side by the perimeter of the circle that defines the track. In other words, the data sector is a storage segment along the length of a track. Generally, a certain number of spare data sectors are included in the storage medium. These spare data sectors may be utilized as replacement sectors for defective data sectors.
Some defective data sectors are formed at the time of disc manufacture. However, defects can arise in any of the data sectors at various times during the lifetime of the storage system (grown defects). Grown defects include, for example, invading foreign particles which become embedded onto the surface of the disc, or external shocks to the storage system which can cause the transducer to nick or crash onto the surface of the disc. Defective data sectors pose either temporary or permanent data retrieval problems.
Read errors are typically determined when the host computer attempts to retrieve user data from a data sector and one or more uncorrected errors exist. In general, each data sector includes data and some sort of error correction for correcting the data in the data sector. In addition, typically, the data storage system includes internally programmed error recovery routines such that upon determination of a read error, the data storage system applies a variety of corrective operations to recover user data. Occasionally, the data storage system exhausts all available corrective operations for recovery of data from a data sector without success. The data storage system will declare a hard error and reallocate the sector by mapping out the bad data sector and substituting an unused, spare sector. If the affected data track is allocated with spare data sectors, then the data storage system can also apply a slip operation to data sectors on a data track. Slipping is the process of moving the data in the bad data sector and all remaining data sectors that follow the bad data sector towards the spare sectors on the data track. The bad data sector is then marked as unusable.
Recently, there has been a rapid increase in the production of smaller disc drives in the disc drive industry. These smaller sized disc drives are for use with handheld or portable devices, such as cell phones, personal digital assistants and digital music players. In addition, overtime, data sector size has been continually increasing to provide greater detection of defective data and better correction of defective data in each data sector. For example, data sector size has increased from approximately 512 bytes per sector to a size of approximately 4,096 bytes per sector. A defect may only occur on a small portion of large sized data sectors. Having to reallocate an entire data sector because a portion of it is defective penalizes the rest of the perfectly good data in the data sector. In addition, finding available spare data sectors on smaller disc drives is difficult because of the limited amount of space available to reserve as spare data sectors.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.